In order to impart anti-corrosion properties to various metal base materials, surface treatments have thus far been performed. Particularly, a zinc phosphate treatment has been generally employed on metal base materials which constitute automobiles. However, this zinc phosphate treatment has a problem of sludge generation as a by-product. Accordingly, a surface treatment without use of zinc phosphate for a next generation has been demanded, and a surface treatment with zirconium ion is one of such treatments (see, for example, Patent Document 1).
Meanwhile, metal base materials which constitute automobiles and necessitate high anti-corrosion properties are subjected to cation electrodeposition coating following the surface treatment. The cation electrodeposition coating is carried out on the grounds that the coated film obtained by cation electrodeposition coating has superior in anti-corrosion properties, and it has “throwing power”, generally referred to, that is a property of allowing automobile bodies having a complicated shape to be completely coated.
However, it has been recently proven that when a metal base material that had been surface treated with the zirconium ion is subjected to cation electrodeposition coating, there may be a case in which the throwing power is not significantly achieved depending on the type thereof. In particular, such a tendency has been revealed to be marked in the case of cold-rolled steel plates. Accordingly, when the cation electrodeposition coating is carried out, sufficient anti-corrosion properties cannot be attained unless throwing power is exhibited.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2004-218070